I expected this for the first season and over the winter following replastering, but this is the 3rd season and I still can't keep it down. I had checked it over the winter a few times before the pool froze and added acid several times, but some time between the freeze and spring start up (last week) the pH must have gone through the roof. The color on my taylor test kit was a really dark purple - I'm guessing probably even up to 9. It took many additions of acid to bring it down, but it seems to go right back up again. The other issue is that I developed some scale over the winter and so, not thinking to check on compatibility first, I went ahead and ordered 2 quarts of Bioguard Scale Inhibitor. THEN I find out that it is not compatible with my biguanide pool, so I have 2 quarts of the stuff that I no idea what to do with. I was told to use SoftSwim Stain Control instead (even though the primary purpose for this is to treat or prevent metal staining).
In talking to BioGuard, I was told that the plaster can still continue to generate high pH even after 2-3 years of replastering, so I guess that's the situation I'm running into. Hopefully it'll settle down soon.
I assume the forum does not have a buy / sell feature, since I'd be happy to list the Scale Inhibitor!